The present application claims priority under 35 U.S.C. xc2xa7119 of German Patent Application No. 199 01 400.0, filed on Jan. 15, 1999, the disclosure of which is expressly incorporated by reference herein in its entirety.
1. Field of the Invention
The invention relates to processes and the corresponding devices for drying and smoothing a web of fibrous material, particularly a paper web, in a machine for manufacturing and/or finishing such a web, including at least one drier section in which the fibrous material web is conducted through a plurality of heated cylinders and a calendar with a number of smoothing nips.
2. Discussion of Background Information
In order to achieve a moisture cross-profile which is as uniform as possible, fibrous material webs have typically been subjected to intense drying and then moistened again at the end of the drying process. This requires a great deal of energy and long dryer sections.
Conventional SC papers achieve a dry content of, e.g., 97-98% at the end of the dryer section, given a good moisture cross-profile of 2 sigma of approximately 0.15-0.3%. With the application of moisture at the end of the dryer section, the dry content here is reduced to 91-93%. After the paper roll has experienced a xe2x80x9cmaturation time,xe2x80x9d during which the moisture content equalizes itself over the thickness of the paper, the paper is fed to a calendar.
To make the manufacturing process more effective, however, there is a growing interest in allowing the calendering process to immediately succeed the drying process. With the elimination of the above-mentioned xe2x80x9cmaturation time,xe2x80x9d however, the moisture cross-profile of the fibrous material web worsens, particularly at high speeds.
Here, the moisture cross-profile measured at the end of the dryer section is used to control the treatment device at the end of the dryer section and is also used, if need be, to control any steam-blowing boxes present in a pressing section positioned in front of the dryer section. This method is not only imprecise, but is also associated with long reaction times, which in turn increases the number of rejects.
The present invention therefore provides for a process for drying and smoothing fibrous material webs. The invention also provides for devices for performing the process. Moreover, the invention allows for calendering to occur immediately following the drying process, particularly at high speeds, while guaranteeing an optimal moisture cross-profile.
The process provides for the fibrous material web in the dryer section to be dried to a dry content of at most approximately 93% and, in the dryer section, the moisture cross-profile of the fibrous material web is influenced a number of times and/or the moisture distribution is made as uniform as possible over the thickness of the fibrous material web by moistening or drying at least one side of the fibrous material web at a plurality of points during the drying process. A jet humidifier or a steam blow box may be used to add the moisture to the web and this device may be located anywhere along the dryer section. Preferably, the influencing device is located towards the end of the dryer section. Moreover, a dryer section having a beginning, middle, and end section should have at least one influencing device in the end section and one or more such devices in the beginning and middle sections.
A significant savings in energy can be achieved by limiting the dry content of the fibrous material web in the dryer section, which preferably will equal a maximum of approximately 91%.
The assurance of a good and uniform moisture cross-profile, as viewed crosswise to the fibrous material web, is achieved with the treatment of the fibrous material web at a plurality of points within the dryer section.
Furthermore, a relatively uniform moisture distribution over the thickness of the fibrous material web during the drying process is advantageous in that limited moistening or drying at the end of the dryer section is sufficient. Consequently, achieving a good moisture cross-profile also does not require excessive drying.
The invention allows at least the moisture distribution over the thickness of the fibrous material web and/or the moisture cross-profile in the first two thirds of the dryer section to be influenced. It is most important for the influencing device (e.g., a jet humidifier or a steam blow box) to be at least applied in the last third of the dryer section. An additional device can also be located between the dryer section and the calender. In agreement with this suggested configuration of the points of influence, but also independent of them, it can be advantageous for influence to be exerted at a moisture content of between approximately 50% and 15%, preferably between approximately 25% and 15%, and, furthermore, at a moisture content of between approximately 20% and 8%, preferably between approximately 15% and 8%.
As a result of controlling good moisture distribution, and/or a good moisture cross-profile at the end of the dryer section, it is possible for the fibrous material web to pass through the dryer section and the calender at a speed of more than approximately 1200 m/min, preferably more than approximately 1500 m/min. It is of special advantage here for the fibrous material web to be heated and to be pressed and steamed across the width of the web in a manner that permits control according to zone.
The intensity of moistening or drying should be adjustable at several points, so as to influence the moisture cross-profile according to zones across the width of the fibrous material web. In each case, the zones should be adjusted on the basis of the subsequent measurement of the moisture cross-profile. The zone widths should be as small as possible and less than approximately 50 mm, preferably less than approximately 25 mm.
The corresponding device provides for at least one influencing device to be assigned to the fibrous material web in at least one of the first two thirds of the dryer section, as well as in the last third of the dryer section, and in the area between the dryer sections and the calender; and/or that at least one influencing device is assigned to the fibrous material web in an area in which it has a moisture content between approximately 50% and 15%, preferably between approximately 25% and 15%, and in at least one area in which it has a moisture content between approximately 20% and 8%, preferably between approximately 15% and 8%. As an end result, at least two influencing devices are employed, i.e., one in the end section and another at any location described above.
To assure as fine a water distribution as possible, the influencing devices (i.e., jet humidifiers) for moistening, particularly in the last third of the dryer section, should spray water on the fibrous material web with as small a droplet size as possible, in range of less than approximately 100 xcexcm, preferably less than approximately 80 xcexcm.
Furthermore, the quotient of the length of the fibrous material web between the last point at which the moisture distribution over the thickness of the web is influenced and/or the moisture cross-profile is influenced, particularly by moistening, and the first smoothing nip of the adjacent calender divided by the speed of the fibrous material web should amount to at least approximately one second. This provides a minimal penetration time and allows the sprayed water to penetrate the fibrous material web up to the beginning of the smoothing process.
Assigned to the influencing devices here are moisture cross-profile measuring units which help control the influencing devices.
To assure a quick reaction time, or optimal control, each influencing device should have a moisture cross-profile measuring unit assigned to it whenever possible, preferably positioned after it in the direction of flow.
It is also possible to provide several moisture cross-profile measuring units and to assign one or two influencing devices to each moisture cross-profile measuring unit.
The moisture distribution over the thickness and width of the fibrous material web can be advantageously further influenced if one of the pressing sections located in front of the dryer section and employed for removing water contains at least one steam-blowing box for the purpose of heating the web that runs at a right angle to the fibrous material web and can be controlled according to zones.
To achieve good values for luster and smoothness in the fibrous material web, it is essential for the calender to include at least three smoothing rollers which, taken in pairs, form one smoothing nip; here, at least one smoothing roller is heated and at least one smoothing roller can be controlled with respect to bending. Moistening within the calender can also improve the result; heating and/or moistening (primarily with steam) will preferably be controlled according to zone.
In the dryer section in which, for the most part, only one side of the fibrous material web comes into contact with heated cylinders, it is generally sufficient for influencing devices to be predominantly assigned only to this one side of the web, preferably for moistening. Assigning at least one influencing device to each side of the fibrous material web may only be necessary in the last third of the dryer section in order to dry the outer areas of the fibrous material web.
If both sides of the fibrous material web come into contact with heated cylinders to any significant degree, then both sides should be assigned influencing devices for moistening.
The invention provides a process for drying and smoothing a web in an apparatus including at least one dryer section having a plurality of heated cylinders and a calender having a plurality of smoothing nips, the process including guiding the web over the plurality of heated cylinders in the at least one dryer section, drying the web in the at least one dryer section to a maximum dry content of approximately 93%, adjusting a moisture cross-profile of the web in the dryer section, and guiding the web to the calender. The web may be a fibrous material paper web. The web may be dried to a dry content of approximately 91%. The process may further include adjusting a moisture distribution over a thickness of the web during the drying by moistening at least one side of the web. The moistening may include a plurality of moistening points which provide the web with approximately uniform moisture content. The adjusting may further include influencing the moisture cross-profile within a first two thirds of the dryer section, and influencing the moisture cross-profile in a last third of the dryer section. The adjusting may include influencing the moisture cross-profile within a first two thirds of the dryer section, and influencing the moisture cross-profile between the dryer section and the calender. The adjusting further may include influencing the moisture cross-profile within a first two thirds of the dryer section, and influencing the moisture cross-profile in a last third of the dryer section and between the dryer section and the calender. The adjusting may include influencing the web moisture cross-profile at a point where the web has a moisture content between approximately 50% and 15%. The moisture content is between approximately 25% and 15%. The adjusting may also include influencing the web moisture cross-profile at a point where the web has a moisture content between approximately 20% and 8%. The moisture content is between approximately 15% and 8%. The guiding may include running the web through the dryer section at a speed greater than approximately 1200 m/min. The running may be at a speed greater than approximately 1500 m/min. The guiding may include allowing the web to absorb moisture for at least one second after influencing and before the web enters the calender. The guiding may include heating the web in the calender and pressing the web over its width according to controllable zones. The adjusting may provide for influencing the moisture cross-profile of a plurality of zones across the width of the web. The adjusting of each zone may be continuous. The adjusting may further include measuring the moisture cross-profile of the web using at least one measurement device. The measuring may be performed at a point which is ahead of a point of adjusting. The measuring may be performed by zone. The adjusting may be performed by zone. Each zone may be adjusted in response to a corresponding zone measurement. Each zone may be continuously adjusted in response to a corresponding continuous zone measurement.
According to another aspect of the invention, there is provided a device for drying and smoothing a web, including a dryer section comprising beginning, middle, and end sections, a calender, and at least one adjusting device located in the end section for moistening the web, wherein the web is guided through the dryer section prior to being guided to the calender. The device may further include at least one adjusting device disposed in one of the beginning and the middle sections for moistening the web. One of the adjusting devices may be located in a section where the web has a moisture content between approximately 50% and 15%. The moisture content may be between approximately 25% and 15%. Another adjusting device may be located in a section where the web has a moisture content between approximately 20% and 8%. The moisture content may be between approximately 15% and 8%. At least one adjusting device may be located in each of the beginning and middle sections. The at least one adjusting device may moisten the web with a fine mist spray. The fine mist spray may be water. The fine mist spray may have droplets which are in a range less than approximately 100 xcexcm. The droplets may be in the range of less than approximately 80 xcexcm. The device may also include a plurality of adjusting devices for moistening the web in zones. The plurality of zones may be arranged across the width of the web so as to define a zone moistening width. Each zone may be moistened by a corresponding adjusting device. Each zone moistening width may be less than approximately 25 mm. Each zone moistening width is less than approximately 15 mm. At least one moisture cross-profile measuring unit for influencing the at least one adjusting device can be included. The device can also include a plurality of moisture cross-profile measuring units, and a plurality of adjusting devices, wherein each measuring unit influences a corresponding adjusting device. The measuring units may comprise infrared measuring devices. Each infrared measuring unit can control a amount of water delivered to the web by each corresponding adjusting device. Each adjusting device may deliver water to a specific zone width of the web. Each adjusting device may be located along a web travel path such that it is followed by a corresponding moisture cross-profile measuring unit. The device may further have a plurality of moisture cross-profile measuring units, and a plurality of adjusting devices, wherein each measuring unit influences a corresponding adjusting device and wherein at least one measuring unit influences at least two adjusting devices. The at least one measuring unit which influences the at least two adjusting devices may be located in the end section. The device may utilize at least one steam-blowing box located ahead of a press nip, wherein the press nip is located before the dryer section. The at least one steam-blowing box comprises a device for heating the web. The at least one steam-blowing box comprises a moisture removing device. The calender may further include at least three smoothing nips arranged in pairs with at least one roller of each pair comprising a heated roller and at least one opposing roller of each pair comprising a bending roller. The dryer section may further include a plurality of heated cylinders, and the at least one influencing device positioned downstream of one of the heated cylinders, wherein the web is moistened by the at least one adjusting device after it is heated by one of the heated cylinders. The device may have at least one aspirating cylinder located downstream from one of the heated cylinders, wherein the at least one adjusting device is located adjacent the at least one aspirating cylinder. The device may also employ the at least one adjusting device being located on one side of the web so as to moisten this one side. The at least one adjusting device may be located on each side of the web so as to moisten the web from both sides. The at least one adjusting device may be at least two adjusting devices of each being located on each side of the web so as to moisten the web from both sides, wherein these adjusting devices are each positioned in end section. The dryer section may have a plurality of heated cylinders, and at least two influencing devices positioned downstream of one of the heated cylinders, wherein the web is moistened by from each side by at least one influencing device after it is heated by one of the heated cylinders. The heated cylinders may be arranged so as to heat both sides of the web.
According to still another aspect of the invention, there is provided a process for drying and smoothing a web, including guiding the web through at least one nip press, guiding the web through at least one dryer section, said dryer section comprising a beginning, middle, and end section wherein the at least one dryer section further comprises a plurality of heated cylinders, measuring a moisture cross-profile of the web with a measurement device located in one of the middle section and the end section, adjusting a moisture cross-profile of the web in one of the beginning, middle, and end section based upon the measuring, and drying the web in the at least one dryer section to a maximum dry content of approximately 93%, further guiding the web to a calender comprising a plurality of smoothing nips.
The invention also provides for a device for drying and smoothing a web, including a first nip press, a dryer section comprising beginning, middle, and end sections, said dryer section further comprising a plurality of heated cylinders, at least one adjusting device for moistening the web located in the end section and at least one of the beginning and middle sections, at least one measuring device located downstream from one of the at least one adjusting device for measuring the moisture cross-profile of the web, and a calender section comprising a plurality if nip presses, wherein the web is guided from the first nip press, through the dryer section, and through the calender section and wherein the moisture cross-profile of the web is adjusted continuously based upon a measured cross-profile.
Other exemplary embodiments and advantages of the present invention may be ascertained by reviewing the present disclosure and the accompanying drawing.